Add compatibility tests

java-compatibility-validator/build.gradle.kts

@@ -0,0 +1,47 @@
+@file:Suppress("UNUSED_VARIABLE")
+
+plugins {
+    kotlin("jvm")
+    java
+}
+
+description = "Java compatibility validator for mirai-core and mirai-core-qqandroid"
+
+repositories {
+    mavenCentral()
+    jcenter()
+}
+
+kotlin {
+    sourceSets {
+        all {
+            languageSettings.enableLanguageFeature("InlineClasses")
+            languageSettings.useExperimentalAnnotation("kotlin.Experimental")
+        }
+
+        main {
+            dependencies {
+                api(kotlin("stdlib"))
+                api(project(":mirai-core-qqandroid"))
+            }
+        }
+
+        test {
+            dependencies {
+                api(kotlin("stdlib"))
+                api(kotlin("test"))
+                api(kotlin("test-junit"))
+                api(project(":mirai-core-qqandroid"))
+            }
+        }
+    }
+}
+
+tasks.withType<org.jetbrains.kotlin.gradle.tasks.KotlinCompile> {
+    kotlinOptions.jvmTarget = "1.8"
+}
+
+java {
+    sourceCompatibility = JavaVersion.VERSION_1_8
+    targetCompatibility = JavaVersion.VERSION_1_8
+}

java-compatibility-validator/src/main/java/test/MPPCompatibilityTest.java

@@ -0,0 +1,20 @@
+package test;
+
+import net.mamoe.mirai.Bot;
+import net.mamoe.mirai.contact.Contact;
+import net.mamoe.mirai.contact.QQ;
+
+/*
+ * Copyright 2020 Mamoe Technologies and contributors.
+ *
+ * 此源代码的使用受 GNU AFFERO GENERAL PUBLIC LICENSE version 3 许可证的约束, 可以在以下链接找到该许可证.
+ * Use of this source code is governed by the GNU AGPLv3 license that can be found through the following link.
+ *
+ * https://github.com/mamoe/mirai/blob/master/LICENSE
+ */
+public class MPPCompatibilityTest {
+    public static void main(String[] args) {
+        final QQ next = Bot.getInstances().get(0).getFriends().iterator().next();
+        ((Contact) next).sendMessage("");
+    }
+}